A conventional composite semiconductor switching device is configured, as described in Patent Document 1 listed below, so that a metal oxide semiconductor field effect transistor and an insulated gate bipolar transistor are parallelly connected in a switch circuit performing power conversion by switching operations, and the metal oxide semiconductor field effect transistor has a gate threshold voltage lower than a gate threshold voltage of the insulated gate bipolar transistor. That is, by parallelly connecting the IGBT and the MOSFET, a small current flows through the MOSFET having a saturation voltage lower than the IGBT, an intermediate current flows through both of the IGBT and the MOSFET, and a large current flows through the IGBT having a saturation voltage lower than the MOSFET. According to the composite semiconductor switching device, its turn-on saturation voltage is that of the MOSFET in a small current range, and is that of the IGBT in a large current range; therefore, in a whole current range, the composite semiconductor switching device has a saturation voltage lower than those of the MOSFET component and the IGBT component to have a smaller on-state loss, thereby being improved in a conversion efficiency.
Another conventional composite semiconductor switching device includes, as described in Patent Document 2 listed below, switching transistors connected in parallel for providing a current to a load, and a pulse generator providing, in response to said current, pulse width modulated pulse cycles each of which has a pulse signal, and an alternate selector controlling, in each pulse cycle, a predetermined transistor to turn ON prior to the other transistors, thereby dissipating all the turn ON losses, and controlling a predetermined transistor to turn OFF later than other transistors, thereby dissipating all the turn OFF losses.
In the composite semiconductor switching device mentioned above, the switching loss can be equally assigned to the parallelly connected MOSFET transistors.